DSL systems have gained an ever growing importance in digital communication and data transfer as the use of internet is expanding to all area of business and home applications. With the introduction of new applications requiring the transmission of a great amount of data a demand for broadband transmission has emerged. Transmission rate and reliability have increased in recent years and have to be improved further in order to comply with future requirements of digital communication systems. Signal to Noise Ratio (SNR) on communication lines is one of the factors that influence broadband capabilities. Crosstalk is usually a major noise generating factor in modern DSL systems such as ADSL and VDSL.
Due to for instance the imperfections in the cable, signals on one line can couple to another line resulting in an increased noise in the received signal on this line. This crosstalk between lines will when the signals are transmitted in the same direction result in far-end crosstalk (FEXT). Crosstalk signals coupled in from a transmitter to a receiver at the same side is called near-end crosstalk (NEXT). This is illustrated in FIG. 2.
For improving the broadband capabilities of communication cables and increasing the transmission rates in both downstream and upstream directions, it is vital to reduce the transmit power in order reduce the FEXT and NEXT which are the main noise generating factors. There have been many attempts for the mitigation of crosstalk. As crosstalk cancellation can not be done in each scenario, dynamic spectrum management (DSM) is an effective way of eliminating the negative impact of crosstalk.
Traditional spectrum management is done in a static way by Static Spectrum Management (SSM). Based on standardized methods to describe DSL-channels the spectral power distribution is setup once in the initialization phase of service provisioning. This setup is then kept during the complete service session. Dynamic spectrum management (DSM) tries to utilize and track the present channel conditions in order to exploit the transmission capacities in an optimum way. The basic algorithms are described by Wei Yu, George Ginis and John M. Cioffi: “Distributed Multiuser Power Control for Digital Subscriber Lines (IEEE Journal on Selected Areas in Communications, Special Issue on Twisted Pair Transmission, vol. 20, no. 5, pp. 1105-1115. June 2002) and by Raphael Cendrillon and Marc Moonen in “Iterative Spectrum Balancing for Digital Subscriber Lines (IEEE International Communications Conference (ICC), Seoul, May 2005). Raphael Cendrillon and Marc Moonen, Jan Verlinden and Tom Bostoen, Wei Yu have described “Optimal Spectrum Balancing for Digital Subscriber Lines (IEEE Transactions on Communications, pages 922-933, vol. 54, no. 5, May 2006.) By smart adjustments of the transmission parameters the system rate, reach and therefore coverage can be substantially increased. Other possible enhancements are higher line/system robustness by providing larger SNR-margins. Depending on the DSM-algorithm used different channel information is needed. For most cases the magnitude of the FEXT-transfer function is sufficient but also crucial.
In order to make use of a DSM algorithm for the reduction of crosstalk, the FEXT/NEXT transfer function has to be determined in an automatic, time and cost efficient way.
Traditionally the FEXT/NEXT transfer function will be determined by using dedicated devices (a signal generator and a network analyzer) on both cable ends. This is easily done as long the cable is on a drum and the person carrying out the measurement has access to both cable ends. In practical cases however, the cable is very long and the cable ends are not accessible either. One cable end is typically in a central office (CO) and the other cable end is at the user side connected to a customer premises equipment (CPE). When performing a traditional transfer function measurement, both cable ends have to be disconnected from the DSL modems and connected to the dedicated devices. During measuring, communication on the tested lines or disconnected cable will not be possible. All steps of measurement have to be controlled and synchronized from either side in order to obtain the required transfer function. An additional difficulty arises from the fact that the user/customer equipments are distributed in different rooms, buildings, streets, districts or cities. Therefore only very few lines can be measured in a cost efficient way. Measuring on many or all lines would require a very long time which makes this approach practically not feasible.
It is commonly known that operators have no access to FEXT/NEXT-transfer function information about their installed cable binders. This is quite natural, since measurements of these qualities can not be done automatically, according to our knowledge. Hence, they can only be made manually. However, this is extremely expensive because it                is time consuming        requires an educated technician with suitable equipment        needs physical access to lines on customer side in addition to the central office        is non-automatic        does not allow to track changes in the transfer function over time and deployment        involves complicated maintenance of storing information (database handling).        
U.S. Pat. No. 6,205,220 suggest a method and apparatus for reducing the near-far crosstalk interference between channels in a communication system. Channels of different lengths that are disposed adjacent to each other and carrying signals at the same frequencies often create cross-talk interference in their neighbouring channels. By spectrally shaping the signals carried on shorter lines the amount of cross-talk interference generated by these lines to longer lines may be significantly reduced, resulting in a better overall performance. This may be accomplished by including spectrally shaping a signal carried on a first channel to reduce the amount of cross-talk coupling to a neighbouring second channel. The shaping of the signal carried on the first channel may also be based in part on characteristics of the second and/or first channel. The characteristics include for example the length of the channels and the transfer functions of the channels. This method concentrates on the problem of non-uniform far-end crosstalk, often referred to as “near-far FEXT” or “unequal-level FEXT” and makes use of DSM on the basis of transfer functions but the way of determining the transfer function is not addressed.
WO2005/114861 suggests the use of operational data to determine the FEXT interference induced by one line into the other DSL line. FEXT interference can be calculated using the NEXT interference measured between the two lines at the upstream ends of the loops and the downstream channel transfer function of one of the loops. Because the NEXT and transfer function constitute a linear time-invariant system, as does the FEXT interference between the lines, the NEXT interference and line transfer function can be multiplied (if in linear format) or added (if in logarithmic format) to approximate the FEXT interference between the lines. This method does not require the lines to be disconnected or the normal operation to be interrupted during measurement, but does not provide a direct measurement of FEXT transfer function, therefore the measured parameters provide only an estimated parameter of low accuracy.
Therefore it is an object of the invention to provide a method for determining the FEXT/NEXT transfer function automatically in a time and cost efficient way. A further object of the invention is to provide a method that makes it possible to determine the FEXT/NEXT transfer function by means of using for example the standardized Loop diagnostic protocol in ITU-T G.992.3/5. It is also an object of the invention to provide a measuring method that eliminates the need for use of dedicated devices.